diff --git a/README.version b/README.version
index 65e27d6..8a3537a 100644
--- a/README.version
+++ b/README.version
@@ -5,3 +5,6 @@
 - ab3390a external/libgav1,cosmetics: add license headers
 - backport cl/281117442: Fully use the frame border for reference block.
 - backport cl/289984918: convolve: Use the correct subsampling for ref frames
+- backport cl/289966078: Move initial_display_delay out of OperatingParamet
+- backport cl/290784565: Handle a change of sequence header parameters.
+- backport cl/291222461: Disallow change of sequence header during a frame.
diff --git a/libgav1/src/decoder_impl.cc b/libgav1/src/decoder_impl.cc
index 9448247..11f2682 100644
--- a/libgav1/src/decoder_impl.cc
+++ b/libgav1/src/decoder_impl.cc
@@ -50,6 +50,27 @@
 
 }  // namespace
 
+void DecoderState::UpdateReferenceFrames(int refresh_frame_flags) {
+  for (int ref_index = 0, mask = refresh_frame_flags; mask != 0;
+       ++ref_index, mask >>= 1) {
+    if ((mask & 1) != 0) {
+      reference_valid[ref_index] = true;
+      reference_frame_id[ref_index] = current_frame_id;
+      reference_frame[ref_index] = current_frame;
+      reference_order_hint[ref_index] = order_hint;
+    }
+  }
+}
+
+void DecoderState::ClearReferenceFrames() {
+  reference_valid = {};
+  reference_frame_id = {};
+  reference_order_hint = {};
+  for (int ref_index = 0; ref_index < kNumReferenceFrameTypes; ++ref_index) {
+    reference_frame[ref_index] = nullptr;
+  }
+}
+
 // static
 StatusCode DecoderImpl::Create(const DecoderSettings* settings,
                                std::unique_ptr<DecoderImpl>* output) {
@@ -173,7 +194,7 @@
         return status;
       }
     }
-    UpdateReferenceFrames(obu->frame_header().refresh_frame_flags);
+    state_.UpdateReferenceFrames(obu->frame_header().refresh_frame_flags);
     if (obu->frame_header().show_frame ||
         obu->frame_header().show_existing_frame) {
       if (displayable_frame != nullptr) {
@@ -195,11 +216,11 @@
             obu->frame_header().refresh_frame_flags == 0) {
           // If show_existing_frame is true, then the current frame is a
           // previously saved reference frame. If refresh_frame_flags is
-          // nonzero, then the UpdateReferenceFrames() call above has saved the
-          // current frame as a reference frame. Therefore, if both of these
-          // conditions are false, then the current frame is not saved as a
-          // reference frame. displayable_frame should hold the only reference
-          // to the current frame.
+          // nonzero, then the state_.UpdateReferenceFrames() call above has
+          // saved the current frame as a reference frame. Therefore, if both
+          // of these conditions are false, then the current frame is not
+          // saved as a reference frame. displayable_frame should hold the
+          // only reference to the current frame.
           assert(displayable_frame.use_count() == 1);
           // Add film grain noise in place.
           film_grain_frame = displayable_frame;
@@ -665,16 +686,4 @@
   }
 }
 
-void DecoderImpl::UpdateReferenceFrames(int refresh_frame_flags) {
-  for (int ref_index = 0, mask = refresh_frame_flags; mask != 0;
-       ++ref_index, mask >>= 1) {
-    if ((mask & 1) != 0) {
-      state_.reference_valid[ref_index] = true;
-      state_.reference_frame_id[ref_index] = state_.current_frame_id;
-      state_.reference_frame[ref_index] = state_.current_frame;
-      state_.reference_order_hint[ref_index] = state_.order_hint;
-    }
-  }
-}
-
 }  // namespace libgav1
diff --git a/libgav1/src/decoder_impl.h b/libgav1/src/decoder_impl.h
index 4e4c6b5..e58a0da 100644
--- a/libgav1/src/decoder_impl.h
+++ b/libgav1/src/decoder_impl.h
@@ -41,6 +41,13 @@
 };
 
 struct DecoderState {
+  // Section 7.20. Updates frames in the reference_frame array with
+  // current_frame, based on the refresh_frame_flags bitmask.
+  void UpdateReferenceFrames(int refresh_frame_flags);
+
+  // Clears all the reference frames.
+  void ClearReferenceFrames();
+
   ObuSequenceHeader sequence_header = {};
   // If true, sequence_header is valid.
   bool has_sequence_header = false;
@@ -64,8 +71,8 @@
   //
   // NOTE: When show_existing_frame is false, it is often more convenient to
   // just use the order_hint field of the frame header as OrderHint. So this
-  // field is mainly used to update the state_.reference_order_hint array in
-  // DecoderImpl::UpdateReferenceFrames().
+  // field is mainly used to update the reference_order_hint array in
+  // UpdateReferenceFrames().
   uint8_t order_hint = 0;
   // reference_frame_sign_bias[i] (a boolean) specifies the intended direction
   // of the motion vector in time for each reference frame.
@@ -118,9 +125,6 @@
   // is handled in Tile::DecodeBlock().
   void SetCurrentFrameSegmentationMap(const ObuFrameHeader& frame_header,
                                       const SegmentationMap* prev_segment_ids);
-  // Section 7.20. Updates frames in the state_.reference_frame array with
-  // state_.current_frame, based on the refresh_frame_flags bitmask.
-  void UpdateReferenceFrames(int refresh_frame_flags);
 
   Queue<EncodedFrame> encoded_frames_;
   DecoderState state_;
diff --git a/libgav1/src/obu_parser.cc b/libgav1/src/obu_parser.cc
index 644f1a5..ca251fa 100644
--- a/libgav1/src/obu_parser.cc
+++ b/libgav1/src/obu_parser.cc
@@ -4,6 +4,7 @@
 #include <array>
 #include <cassert>
 #include <climits>
+#include <cstddef>
 #include <cstdint>
 #include <cstring>
 
@@ -60,6 +61,15 @@
 
 }  // namespace
 
+bool ObuSequenceHeader::ParametersChanged(const ObuSequenceHeader& old) const {
+  // Note that the operating_parameters field is not compared per Section 7.5:
+  //   Within a particular coded video sequence, the contents of
+  //   sequence_header_obu must be bit-identical each time the sequence header
+  //   appears except for the contents of operating_parameters_info.
+  return memcmp(this, &old,
+                offsetof(ObuSequenceHeader, operating_parameters)) != 0;
+}
+
 // Macros to avoid repeated error checks in the parser code.
 #define OBU_LOG_AND_RETURN_FALSE                                            \
   do {                                                                      \
@@ -247,7 +257,7 @@
   return true;
 }
 
-bool ObuParser::ParseSequenceHeader() {
+bool ObuParser::ParseSequenceHeader(bool seen_frame_header) {
   ObuSequenceHeader sequence_header = {};
   int64_t scratch;
   OBU_READ_LITERAL_OR_FAIL(3);
@@ -307,8 +317,7 @@
         OBU_READ_BIT_OR_FAIL;
         if (static_cast<bool>(scratch)) {
           OBU_READ_LITERAL_OR_FAIL(4);
-          sequence_header.operating_parameters.initial_display_delay[i] =
-              1 + scratch;
+          sequence_header.initial_display_delay[i] = 1 + scratch;
         }
       }
     }
@@ -400,7 +409,17 @@
   if (!ParseColorConfig(&sequence_header)) return false;
   OBU_READ_BIT_OR_FAIL;
   sequence_header.film_grain_params_present = static_cast<bool>(scratch);
-  // TODO(wtc): Compare new sequence header with old sequence header.
+  // Compare new sequence header with old sequence header.
+  if (has_sequence_header_ &&
+      sequence_header.ParametersChanged(sequence_header_)) {
+    // Between the frame header OBU and the last tile group OBU of the frame,
+    // do not allow the sequence header to change.
+    if (seen_frame_header) {
+      LIBGAV1_DLOG(ERROR, "Sequence header changed in the middle of a frame.");
+      return false;
+    }
+    decoder_state_.ClearReferenceFrames();
+  }
   sequence_header_ = sequence_header;
   has_sequence_header_ = true;
   // Section 6.4.1: It is a requirement of bitstream conformance that if
@@ -1944,6 +1963,20 @@
         }
       }
     }
+    // Validate frame_header_.primary_reference_frame.
+    if (frame_header_.primary_reference_frame != kPrimaryReferenceNone) {
+      const int index =
+          frame_header_
+              .reference_frame_index[frame_header_.primary_reference_frame];
+      if (decoder_state_.reference_frame[index] == nullptr) {
+        LIBGAV1_DLOG(ERROR,
+                     "primary_ref_frame is %d but ref_frame_idx[%d] (%d) is "
+                     "not a decoded frame.",
+                     frame_header_.primary_reference_frame,
+                     frame_header_.primary_reference_frame, index);
+        return false;
+      }
+    }
     if (frame_header_.frame_size_override_flag &&
         !frame_header_.error_resilient_mode) {
       // Section 5.9.7.
@@ -2296,7 +2329,7 @@
       case kObuTemporalDelimiter:
         break;
       case kObuSequenceHeader:
-        if (!ParseSequenceHeader()) {
+        if (!ParseSequenceHeader(seen_frame_header)) {
           LIBGAV1_DLOG(ERROR, "Failed to parse SequenceHeader OBU.");
           return false;
         }
diff --git a/libgav1/src/obu_parser.h b/libgav1/src/obu_parser.h
index 9d972b1..e973433 100644
--- a/libgav1/src/obu_parser.h
+++ b/libgav1/src/obu_parser.h
@@ -5,6 +5,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <memory>
+#include <type_traits>
 
 #include "src/decoder_buffer.h"
 #include "src/dsp/common.h"
@@ -166,13 +167,25 @@
 };
 
 struct OperatingParameters {
-  uint8_t initial_display_delay[kMaxOperatingPoints];
   uint32_t decoder_buffer_delay[kMaxOperatingPoints];
   uint32_t encoder_buffer_delay[kMaxOperatingPoints];
   bool low_delay_mode_flag[kMaxOperatingPoints];
 };
 
 struct ObuSequenceHeader {
+  // Section 7.5:
+  //   Within a particular coded video sequence, the contents of
+  //   sequence_header_obu must be bit-identical each time the sequence header
+  //   appears except for the contents of operating_parameters_info. A new
+  //   coded video sequence is required if the sequence header parameters
+  //   change.
+  //
+  // IMPORTANT: ParametersChanged() is implemented with a memcmp() call. For
+  // this to work, this object and the |old| object must be initialized with
+  // an empty brace-enclosed list, which initializes any padding to zero bits.
+  // See https://en.cppreference.com/w/cpp/language/zero_initialization.
+  bool ParametersChanged(const ObuSequenceHeader& old) const;
+
   BitstreamProfile profile;
   bool still_picture;
   bool reduced_still_picture_header;
@@ -213,11 +226,30 @@
   bool decoder_model_info_present_flag;
   DecoderModelInfo decoder_model_info;
   bool decoder_model_present_for_operating_point[kMaxOperatingPoints];
-  OperatingParameters operating_parameters;
   bool initial_display_delay_present_flag;
+  uint8_t initial_display_delay[kMaxOperatingPoints];
   bool film_grain_params_present;
+
+  // IMPORTANT: the operating_parameters member must be at the end of the
+  // struct so that ParametersChanged() can be implemented with a memcmp()
+  // call.
+  OperatingParameters operating_parameters;
 };
 
+// Verify it is safe to use offsetof with ObuSequenceHeader and to use memcmp
+// to compare two ObuSequenceHeader objects.
+static_assert(std::is_standard_layout<ObuSequenceHeader>::value, "");
+// Verify operating_parameters is the last member of ObuSequenceHeader. The
+// second assertion assumes that ObuSequenceHeader has no padding after the
+// operating_parameters field. The first assertion is a sufficient condition
+// for ObuSequenceHeader to have no padding after the operating_parameters
+// field.
+static_assert(alignof(ObuSequenceHeader) == alignof(OperatingParameters), "");
+static_assert(sizeof(ObuSequenceHeader) ==
+                  offsetof(ObuSequenceHeader, operating_parameters) +
+                      sizeof(OperatingParameters),
+              "");
+
 // Loop filter parameters:
 //
 // If level[0] and level[1] are both equal to 0, the loop filter process is
@@ -477,7 +509,7 @@
   bool ParseDecoderModelInfo(ObuSequenceHeader* sequence_header);  // 5.5.4.
   bool ParseOperatingParameters(ObuSequenceHeader* sequence_header,
                                 int index);  // 5.5.5.
-  bool ParseSequenceHeader();                // 5.5.1.
+  bool ParseSequenceHeader(bool seen_frame_header);  // 5.5.1.
   bool ParseFrameParameters();               // 5.9.2, 5.9.7 and 5.9.10.
   void MarkInvalidReferenceFrames();         // 5.9.4.
   bool ParseFrameSizeAndRenderSize();        // 5.9.5 and 5.9.6.